The legal situation is as follows: legal tender means that if you owe somebody a debt, they must accept that legal tender to pay that debt. It does not mean that people must accept legal tender for any trade. Terms of trade must be agreed by both parties and may have all kinds of rules in them.

So you can do the trade, somehow fail to pay according the terms of trade. Now you owe them a debt. Now you can pay that debt with legal tender.

Legal minds - please correct me if I'm wrong.