simple pos desktop project

we meed an exe project by C# ( we prefer xamrin) that we can produce apk file , algorithm and design of database are ready
we need the following features
login ( api will provide )
get product ( api will provide)
app can work after get the info without internet
print to a printer
show product and create invoice
return invoice( all algorithm is ready )
this is the phase 1 and we need to be done in 7 days