You should not make such an offer. Either ask a small fee for the project or either get a cool idea and make a small app and post it on github, or maybe you can

You should not make such an offer. Either ask a small fee for the project or either get a cool idea and make a small app and post it on github, or maybe you can start a small startup if you have a lot of time and just want to show people your coding skills. Do not make deals that have an unknown outcome for you