When trying to finance a single invoice, this is called spot factoring. It usually involves a significantly large invoice. Most factoring companies will not do spot factoring due to the risky nature of the transaction. Obviously if there is a problem with the account debtor paying the invoice there will be little or no recourse available. So the first step would be to determine the creditworthiness of the customer who owes on the invoice. Unless they are a household name Fortune 100 company, there’s not going to be much appetite to fund. The next thing a factoring company will look at will be the contract. How ironclad is the language regarding payment for services rendered?
So the main reason factoring companies shy away from spot factoring is there is a lot of work that goes into getting the account started with the client and completing proper due diligence on the client’s customer. A factor would much rather rely on an ongoing relationship with a variety of client customers.