The best thing to do is to give them the amount that wouldn't be a problem to you,so that if they can't pay back,you wouldn't be who worried about it. Another way is to set a trap for them by borrowing them whatever amount of money that they asked. If they can't pay you back,then you can use the previous unpaid debt as an excuse.
These two ideas are good and the first seem to be the general concensus about the way people treat the issue of borrowing money but the second which is letting go of the money they want to borrow as trap but that is dangerous depending on how much is involved and your strength to forfeit it. If the sum is huge, it will be difficult to do as a trap except it is still within your reach to forfeit it. I still prefer the first one to be used especially if you know the person will not be able to repay how much is involved.