Could this be some kind of a scam and the person who sent the transaction did it on purpose so eventually he can get the coins back to his wallet.
Yes.
You should definitely NOT provide ANY product or service until the transaction has at least one confirmation. Once it has a confirmation, it will be pretty safe. If it has two confirmations, you can consider it permanent.
However, as long as it is unconfirmed, there is a possibility (intentionally or accidentally) that the transaction could vanish and you'll be left without the bitcoins at all.
The mining pool at ViaBTC.com has a service that will confirm a transaction in the next block that they solve. You can find that service here:
https://www.viabtc.com/tools/txaccelerator/The service only accepts 100 transactions per hour, so you'll want to try it as close to the top of the hour as you can, before many other people have used it. If it says it has already reached the limit, you can try again every hour until you get in soon enough.
You'll need to first add the transaction ID of the transaction that was sent to your buyer (
a6631206094461b357f2da764eafe1fd3b9385f00813ed3549f76936ea46f92c ). Then once that is confirmed, you can go back and add your transaction ID.
If you are in a hurry, you could also offer to pay a small fee to someone that has access to choose which transactions a mining pool confirmed in exchange for them adding both of these transactions to their next block. There are a few users here at bitcointalk that have that ability. Try contacting the user with the ID: Quickseller
