It is true that with ECOWAS license, which I gues Gambian license plates are, it will be not a big hussle to get a passavant issued. Last year, we tried the same (with a dutch license plate, no carnet de passage), tactics of having patience on our side: setting up our tent. Eating together with the border officials. Telling them we were going to stay for a longer period in Sierra Leone. Telling them we didn't have much money. Ended in paying 150 euro for a passavant of 5 days. Which was (at that time) the lowest they (the border officials) could go.
This doesn't mean it isn't a scam, but atleast it is not a regular scam but well organized with higher officials in St. Louis involved. I had to go by taxi to St. Louis to get some papers stamped and signed by a high military person. Still have the papers. Zargan from the tourist office involved. Drove also a year with Sierra Leonean license: different rules apply.

Cheers

You said it very clearly. Zargan. He takes half and the customs official. These are his fees. Very lucrative business. They laugh their head off. Military person. Yes high customs officials wear uniform too. They take advantage of non french speakers...
The official fee is 5000 cfa. Unless its more than 5 seats. Than its commercial 2500 cfa more. I heard from a friend in Mali that there is indeed convoys from Dakar Port to the border of mali where some higher convoy charges are involved. To avoid cars being lost... and illegally sold in Senegal. Maybe your tactic should be next time to say youre tourist and want to stay a month in Senegal. St. Louis is known to collect bribes. Never had problems in Dakar. But its probably easier with Ecowas plates.
http://www.douanes.sn/fr/node/92
http://wikioverland.org/Senegal
Follow that link. And print it out.
Passavant de circulation. Pour le touristque. I would threaten to call somebody higher up in customs in dakar. I know the french dont take this BS. And no need to use one of these touts. All crooks.
Welcome to Africa
Good luck.
PS I d really curious to see that letter you paid 150 € for. It cant be the passavant. Because thats surrendered upon exit.
Try Rosso next time... :-)

If the new bosses in Dakar are less corrupt then the former, perhaps they should be alerted to the ongoing abuse on the northern frontier.
There is nothing of the sort on the Mali border posts.
Just the usual minor stuff. Like everyone on the bus having to pay 2000CFA at the checkpoints. Except for Ghanaians who paid 5000. And except for me the toubab, who didnt pay anything.

For Mali, contact Casper Jansen (from NL, too) at Coolcampmali. Besides relevant local knowledge he's got a great campsite on the Baffing river.