No you said it yourself the value fluctuates. You could pay $300 in bitcoins, and the next day it might be worth $310 dollars, or $290. Bitcoin price volatility is something that's going to be around for a while yet, until we get into the hundreds of millions and billions of users at which point the pool of bitcoin is spread out so much that volatility essentially averages out to near zero. Merchants can use services like Bitpay, which instantly converts payments made in bitcoin back into dollars at the rate at the time of the sale, but they charge a 1% commission. I don't see the volatility as a problem because do you know what doesn't hold its value? Dollars. If you have $300 today, its worth less than that tomorrow. It still says $300 on it, but it buys you less. The dollar has lost somewhere between 96 and 99% of its value since 1913. So which would you rather use; one that is volatile but trending upwards, or one that is stable but circling the drain?

It would be worth $300 only at the time the transaction locks in the price. However, the price would change as BTC exchanges run 24/7, like a Forex market. BTC's value may move by a lot, or by a little, during the time the seller owns the currency.

"would it still hold a value of $300? " absolutely not: firstly it fluctuates, secondly it has no dollar-"value" only a bitcoin value.

dont be an idiot

So I'm having trouble with this whole bitcoin thing. So say what I wanted to buy was worth $300 USD and the transaction were to be paid with bitcoins. The current rate is about $621 for 1 bitcoin; when I trade said $300 using localbitcoins.com for bitcoins, I get a decimal. I know the price for each bitcoin fluctuates often so my question is if that $300 in coins were given to the seller, would it still hold a value of $300?