Firstly - one thing to point out - the schema I described above represents the aggregate production of a closed system society - the 900 investment, 100 profit, and 1,000 of commodity production represents not the activities of an individual company/capital, but the total social product, which would obviously be split across many individual capitals - for the purposes of reproduction schema marx broke this done into two branches of production. Dept I which produced means of production and Dept II which produced means of consumption.
Ok, so I start the thing off by borrowing 900 groats on terms that say that I will pay the bank 990 after one year. I invest as you say, 500 in materials, 400 in labour, and I sell for 1000 groats. But let us say that the thing my workers are making takes a year to make. So during that year, I also need to borrow extra money in order to consume myself this year.
the premise that the total 1,000 of output takes a year to make is not realistic within the terms of the schema - remember the 1,000 is the total social product - which includes 500 of means of consumption (400 which is ultimately consumed by labour and 100 consumed by capitalists for luxury/consumption). If it took a whole year to finish the production of this means of consumption the working class (and owners of capital) would be extinct as they would be unable to re-produce themselves, and even simple reproduction would not be achieved. So we have to assume that at least the means of consumption are produced evenly throughout the period - to enable the working class (and capitalists) to reproduce itself and be able to continue to work/appropriate.
So with this in mind, we can go back to your capitalist who needs to consume during the year -
i) our capitalist may be an owner of capital in Dept I which produces means of production - he has enough money at the start of the year for his share of the total 900 investment - however not all of this will be spent upfront - labour is always paid in arrears, not in advance - so he will have a fund of money, earmarked for paying labour for the whole year that is not all used up at once. If he needs to consume, there's no logical reason for him to borrow money to do this as he is already has a money fund available. Say he dips into this to buy his own means of consumption during the year. As he casts that money into circulation to buy means of consumption from capitalists in Dept II - this money will eventually return back to him when capitalists in Dept II have to renew the means of productions used up in their production process, as they have to buy this from capitalists in Dept I (and as we have already established that the means of consumption are produced regularly throughout the year, the capitalists in Dept II who produce this will regularly have to buy new means of production from Dept I to achieve this) - so the money initially cast into circulation by Dept I capital, continually returns to them in this way - no new money is required. This is a key thing that marx noted within a system of reproduction like this (and which is relevant here), that money cast into circulation always returns to its point of origin
ii) our capitalist may be an owner of capital in Dept II which produces means of consumption - as above, he has enough money at the start of the year for his share of the total 900 investment - and as above, not all of this money will be used up front, so he has a pot of money that can be used to fund consumption during the year. Again, there is no logical/rational reason for him to borrow money to fund consumption, when he is already sitting on a fund of money that is available to be used. Additionally, if we assume there are only two capitalists in total, Dept I and Dept II, then the output of production controlled by the capitalist in Dept II is means of consumption in the first place - so abstractly, he doesn't need money to mediate the commodity exchange required for his consumption, he just takes part of his share of appropriated surplus value directly as product - and as mentioned above, it's not possible for this output to not be available until the end of the year as means of consumption must be produced on a regular enough basis to reproduce labour power used up in production. Furthermore as the capitalist in Dept II is the one selling means of consumption to workers, which are paid say weekly/monthly, and must spend most of their income on means of consumption - the money that capitalist in Dept II throws into circulation through paying wages, returns to them within a short time as the working class consume - so again there is no need, at a total social level, for capital to borrow to fund its own consumption
So the rest of your schema doesn't really make too much sense based on what I pointed out above - however i'll respond to it anyway to the extent that its possible
This consumption on credit is funded by an extra loan of 10 groats. At the end of the year, I must pay back 11 groats.
OK, so for the sake of argument say you have to borrow to consume and borrow 10 with 1 interest.
During the barter period, it was my custom to take one tenth of the value of production for my own personal consumption, but I decide to cut back on my consumption this year to get the business kick-started, which leaves 90 groats' worth of unconsumed value somewhere this year, which will have to be carried over into next year – which ought to be allowed, I would have thought: while production may be constant, consumption may vary year by year if the produce is assumed to have a shelf-life.)
I'm not sure I understand this - our simple reproduction schema consists of an ongoing core investment of 900 each year to 'produce the same thing by the same amount of people', and as production (and productivity) is constant that means 100 surplus value is produced each year - giving a total of 1,000 value that can be realised/reinvested by capital. Only 900 of this can be reinvested if we are assuming simple reproduction/no growth, so the remaining 100
has to be consumed by capital in means of consumption/luxury items (which of course are produced by Dept II as part of it's overall commodity output) - this could of course be consumed over a period of more than one year, if that's what you are meaning
So at the end of the year, I must pay the bank 1001 groats, but I only have 1000.
As i stated above, there would be no rational/logical reason for capital to borrow the 10 required for consumption as they would already be sitting on money which could be used to fund this which would return to him before the end of the period anyway (via the back and forward interchange between Dept I and Dept II). Even if we assumed for some bizarre reason capitalists in both Depts didn't use their own money fund (which would be deposited at a bank or hoarded until needed) and instead borrowed the money, they would in fact be borrowing money from each other (or indeed borrowing from themselves) and lending it to each other at the same time which in terms of money required to do this would just negate itself out - i.e. if capitalist in Dept I wanted to borrow, the money to fund that loan wouldn't need to be created/additional money, instead the money that Dept II had on deposit would be used (by the banking system) to do this, and vice versa between Dept II and Dept I. So no new money needs to be created to do this, and the interest that each capitalist had to pay would be offset by the interest they have earned by lending out their surplus money through the banking system (obviously there would be interest margins involved here, but give it's highly unlikely someone with money available would borrow in the first place we can ignore them for the purposes of showing that no increase in the money supply is required even if they did go down this highly unlikely route - edit: in fact the interest margins would probably negate themselves out anyway as capital in Dept I and Dept II would be both borrowing & lending)
I also need to refinance for the next year, which I do by taking out another loan. But this time, instead of borrowing 910 groats for the following year, I have to borrow 911 to pay for the interest on my consumption during the first year.
society needs 900 in total to recommence production at the same level - any consumption by capitalists throughout the year will follow that which I set out above - i.e. it does not need additional borrowing of 10, let alone additional borrowing of 1 being the interest on the 10. And ultimately all capitalists consumption is funded, in value terms, from the surplus value it appropriated during/throughout/at the end of the year - so in total only 900 is required for the next period
(need to split rest of reply over another post as exceeded character limit)