Contra / Set off between debtor and creditor

Hi

Please help am wondering if I have dreamt this!

When you have suppliers that are also customers and you want to 'contra' / 'set off' the suppliers invoice against one we have sent to him as our customer do we have to write to him to advise & formalise this between us - or can I just do it and send him the balance (which we owe him - on this occasion)?

I thought I'd read somewhere of the risks of doing this if our business is subsequently involved in bankruptcy / liquidation etc etc.

Many thanks as always.