Ralph Is there an additional parameter that makes the amount editable (but NOT the description)?
No. If you use the "bunq.me/myname/750/HelloWorld"-style links, then these values aren't fixed and can always be changed. The alternative is generating a payment request and sending the corresponding link to the customer, but then the amount and the description are both fixed.
If you're quite tech savvy you could make a website that allows your customer to input the amount of money that they want to pay, create a payment request according to that with your description of choice on the fly through the API, and then immediately redirect the customer to that request.
Maybe there is a difference between personal and business.
There's no difference in regards to these features.