Gearbox woes

[PB79]

New to forum.

Happy owner of Audi A7 Sportback S line 3.0 T. 67000 miles

Month ago heard odd grind sound, only when accelerating in 5th and dropping to 4th.  Slowing down does not cause the sound.

Audi had it a month.  No warning lights, nothing on diagnostics.  Samples gear box fluid.  Described as murky. They have decided it needs a totally new gearbox.  More than the value of the car.

Full service history with them including gearbox, until last year when it was a basic service and I wanted to give the business to a local garage bouncing back from the pandemic.  Car was out of warranty.

Car is back with me now.

Audi are completely washing their hands of it, entirely based on the last basic service.  Will not negotiate.

Spoken to local gearbox specialist, who without seeing it have said replacement, with refurbished unit.  Again an unaffordable amount given value of the car.

Any insights or suggestions would be very welcome.