l need help quick as l am selling the car and the buyer is trying to knock me down a lot by saying that if the speedo fluctuates it means a lot of work to sort it out, is he telling truth of trying to swindle me

No, just buy a new speedo. It is not a major problem so wait until the next buyer, and get it fixed yourself - it is cheaper than being knocked down a fortune.

Maybe it just needs a new speedo cable, rather than a speedo.
Get another buyer (incidentally - it is illegal to have a speedo that does not work properly, so you need to sort it before selling.)

:-)

so do kangeroos.

Your speedo needs a new cable. about £40 to fit and supply.

The speedometer cable has a square rod on the end of the cable. The square rod goes in a square hole on the back of the speedometer. After a few years, the rod can begin to round off a bit from constant turning & friction.
During high speed turning of the cable, one corner of the square rod, then one ''round corner'' slipping a rotation, will cause irregular movement on the speedometer needle.
Ask ur favourite garage (any, not necessarily Peugeot) to order the Peugeot cable & install it for you.

now it depends on the year of the car some dont have cables
if so you might check the speed sensor at the top of the gearbox as the get covered in fluid and corrode