Repayments

If i have a total figure (that can change) and this total figure is to repay a loan (max 500) and shares (max dictated by another cell's figure) separately. How do I make sure whatever the total figure is, that it pays off the whole loan first (the 500) and the rest goes to the share. And if the total figure is greater then both maximums, the excess is put into another cell (balance b/f). This is an example:

total figure e.g 750 or 250 or 1000

loan repayment e.g 500 250 500
share repayment e.g 250 0 250

balance b/f 0 0 250

Share repayment max 250

I hope this example makes sense.
Thanks so much

Hugo