First of all, reread the original post. @d3mad received an actual sales invoice from the supplier, so a purchase invoice would have been appropriate, as would Receive Money against the Accounts payable account and the supplier’s subaccount. A separate question is whether the supplier was entitled to send that invoice–probably not, since no work had been performed. It should have been a deposit against a quote or contract, but no harm done.
Your confusion about Customer credits and Supplier credits traces to the change announced here:
Those two accounts are no longer accessed directly, but as a result of excesses in Accounts receivable/payable, respectively the absence of specific invoice numbers when transactions are entered.