It could be to drum up business or it could be for real. The folks that purchased the house next to my mom's place picked the neighborhood and had the house style/floorplan they wanted and had their agent cold call the owners. It turned out that my mom's neighbors were looking to sell and move to Florida so it worked out for them.
With respect to commissions. The seller sets the commissions (or that was my experience when we sold our house in San Diego). The commission was listed where buyer agents could see (ie: "2.5% paid to buyers agent" was what we had listed). In your case, the amount of commission you paid the potential buyers agent is something that you can negotiate with them. They could argue that you need to pay a 5% to them, but if it is too high then you can pass. From what you have said, the market has very few options and if there is a buyer out there looking for a 4 bedroom in your neighborhood, and any commission is better than having a buyer move on to a different agent.
My recommendation would be to call the number and find out what the deal is. If he truly has a buyer then have them make an offer with the requested commission as part of the offer. You can always counter with the same offer price and lower commission (if you think the agent is asking for too much). If you have an offer on the table that you like, then you could offer your agent a flat fee rather than a commission to assist, but at this point I don't think you would need one. The contracts are standard and once you have an accepted offer/contract the selling agent does very little that you could not do yourself. You would mostly be dealing with the closing company.
When we sold our house we offered the buyers agent 2.5% and our agent 2% (these were her suggestions and she was fine with the lower commission as we had purchased the house from her...also she was really good and was making bank so 0.5% didn't mean too much to her). After we had a buyer things started to go south a little and the buyer requested a price change. Initially we said no, but our agent negotiated with their agent to reduce the commissions to compensate us for the price drop. In the end the change for us was almost negligible, the buyers got the house, and both agents still got paid without having to continue to advertise or show houses.