Use the PayPal email link instead of the coded link and you can link it to any button or any text. I have to do that all the time for event registrations. My payment buttons have to say specific things, so I create the button and then click the tab for the email link and use that.
Also, if you happen to have another form on your page and you use the PayPal created code, you will have to use the Multiple Forms action; whereas, if you use the email link, you won’t.
Both the code and the email link do exactly the same thing: they take you to the PayPal page to pay/login for that item.