Hi,
take a look at that link because the same phase has been used in the same way:
http://findarticles.com/p/articles/mi_qa3898/is_199810/ai_n8822112If you want to take a credit, you have to offer some security known as loan security or collateral security so the bank could be sure the money would not have gone in the worst case scenario. Sometimes credit against is used in order to name the kind of security. You can see a very good sample for this in the mentioned link above.
Sometimes it is a bit complicated because it means 'to charge up against sth'. You know one person has a claim and the other person has a claim and so they can charge the one claim up against the counter-claim. At least in Germany you can do that but it is a common principle.
In your case I would say that the withholding tax can be used as a credit not from the bank but a form of credit and also charged up against the income taxes, so my understanding of the phrase but maybe Feebs11 have a better idea.
Best wishes,
Kathrin