That would be completely unprofessional on both sides. If you are doing commission work, the person you are paying can simply test the code and then pay. Or sign a contract if both parties are eligible to do so. I don’t want to stray too far off topic, but it’s worth noting that you retain the legal rights to your source code until you legally sign them away. Private modules are not necessary for this.
If you’re paying someone to write you code, they should deliver that code. The notion that you would somehow never give someone access to source code they paid you to write is silly and not sustainable.