Match tax and discount amount from source system

We have a upstream sales platform which will pass the price, tax & discount amount to Fusion.

To avoid fusion's SO amount not matching the upstream sales platform's order amount,

we would like to

1, apply the tax calculated by the source system instead of calculated by Fusion
2, apply the line total discount from source system instead of apply discount in unit price (to avoid rounding problem)

Please advise how can it be done.