How we sync data into our cloud server by using C#

Question

Hi, I am new on this, I need a solution actually,
report is coming from tally to excel and then excel to local SQL by Importing and then Local SQL to our Cloud server.

suppose we installed a software, so first time we will put all records till today from his local SQL to our cloud server.
now suppose client sync data on daily basis by enter date and it will work on our C# code.

Now my question is

1- suppose client make a entry on back date, client's supplier has receipt of back date and client forgot to enter it in invoice table,
so now how we can sync data from local SQL to Cloud Server without any comparison and overwrite in our cloud server.
because it will take more more time.

2- suppose client forgot when last date he had synced data on cloud server in that case also no comparison and no overwrite he wants.
how we can achieve it please give me just solution please.

 

 

please help me my boss give a question to think about it