GPL and it’s derivatives are ones of the most zealous of them, yet they allow you to sell the software. FOSS doesn’t require you to give away software for free and it’s a common misconception. Apache for example also allows you to sell the software.

https://en.m.wikipedia.org/wiki/The_Free_Software_Definition#The_definition_and_the_Four_Freedoms

You only lose monopolistic control over it.

Software is often free, because other people can choose to alter the software and redistribute for free.