Mismatched data types in C# Alpaca library

decimal quantity = client.GetPositionAsync(ticker).Result.Quantity
Quantity here returns a decimal type

_ = client.PostOrderAsync(LimitOrder.Buy(ticker, (long)Quantity, buy_price)).Result
but here is takes a long type

for systems dealing with money, it should stay as a decimal type, so why is one a decimal and one a long?