I interpret that as the minister, having borrowed 200 merks and given an IOU or similar written document, returning what he owed, with interest.
The treasurer then lent 1000 merks to someone else, receiving their IOU and the approval of the Session for the loan.
It's quite common to see in the KS accounts money coming in as interest on bills granted, i.e. loans made by the KS to suitable borrowers.