buying bitcoins question

hello, I am new to bitcoin and last week I have bought my first half a bitcoin through coinbase, everything went ok but my question is: I bought it not so much to keep it as bitcoin at this time but to trade it with another cryptocoin, so my problem is that if I buy with coinbase it takes 3 or 4 days to transfer the money but you pay the bitcoins at the time of purchase, this can be risky because if I buy at a specific price and during the days the money get transferred the bitcoin price drops, I can't sell it or do anything about it and by the time I have my bitcoins they will be worth less, I know there is the possibility for an instant payment but it requires a credit card, and I only have a debit card. Is there any way to transfer usd as usd instead of bitcoins? this way I would have usd and then I can convert them to bitcoins only after they are transferred? it looks like bitstamp does it? can I send US dollars to bitstamp from my american bank account and only after the money are transferred as USD can I convert them to bitcoins? does anyone do it this way? thank you!