I am not sure from the problem statement, actually I don't get something. Suppose that we have this test case
5
1 20
1 0
2 0
3 20
3 0
2
1 5 10
3 2 25

Is answer = 2 for this test case?
4 buy debt from 5 to 1 and now 1 have 30 or it still has 20?
If it has 30, then it can buy debt from 2 to 3.

If this is wrong, can tell me what is the solution to this test case, please? Thanks.

banks may not use the funds they receive after selling their debts

Thanks. I misread that part.

But, there is still a question:
5
1 100000
1 10
1 0
1 0
2 0
2
1 3 10
5 4 1000

If 2 buy debt from 3 to 1, then it will become responsible and 1 can buy debt from 4 to 5 and become responsible again. Will 1 do that for 2? Is answer here 2 or 1? Thanks.

The answer is 2 for that test.