use FtPrecise to convert to contracts and back

freqtrade/exchange/exchange.py

@@ -2891,7 +2891,7 @@ def amount_to_contracts(amount: float, contract_size: Optional[float]) -> float:
     :return: num-contracts
     """
     if contract_size and contract_size != 1:
-        return amount / contract_size
+        return float(FtPrecise(amount) / FtPrecise(contract_size))
     else:
         return amount
 
@@ -2905,7 +2905,7 @@ def contracts_to_amount(num_contracts: float, contract_size: Optional[float]) ->
     """
 
     if contract_size and contract_size != 1:
-        return num_contracts * contract_size
+        return float(FtPrecise(num_contracts) * FtPrecise(contract_size))
     else:
         return num_contracts