Ok to not tax a leasing company with reseller certificate?

One of our customers has engaged a leasing company to pay us, however, the leasing company refuses to pay sales tax (we have nexus in the state customer is located in) as they say they have reseller certificate (which they provided). They want us to list them as the "bill to" and the customer as the "ship to" on the invoice. However, our contract is with the customer. I thought Nexus was based on the "ship to" and not the "bill to". Thoughts? I'd much prefer to be guaranteed payment in full up front, however, I am also focused on compliance. Does the leasing company reseller certificate make this a non-issue? We had similar issue with another leasing company (they didn't have reseller certificate) and were able to charge tax on the invoice after much effort. This company is being a bit adamant about charging double-tax. Any guidance is much appreciated as it is month end and the rep is anxious to close the deal (s). Thank you.