Ok, sold 5 tanks today and recorded the silver figures. It does seem to be linked to equipment.
I sold 2 tanks without equipment (except the Enhanced Target Info) and all the figures for silver added up, and I got what was shown in the log and on screen.
The 3 I sold with equipment all had Advanced Optics, Advanced Concealment, Improved Ventilation, and Enhanced Target Info.
For these 3 I received the figures that popped up briefly in the garage after the sale, and not the amount shown on the sale screen or in the log.
Rather oddly the lowest Tier cheapest tank had the greatest difference of 100,000 silver, and the highest Tier tank had a difference of 26,750
Details Below
Skoda T-50
Silver before sale 280,016,313
On Sale Screen (& in log) 2,754,850
On Garage pop up (amount received) 2,728,100
Silver after sale 282,744,413
IKV 90 B
Silver before sale 282,744,413
On Sale Screen (& in log) 1,439,650
On Garage pop up (amount received) 1,407,850
Silver after sale 284,152,263
Smoothman
Silver before sale 285,376,263
On Sale Screen (& in log) 990,000
On Garage pop up (amount received) 890,000
Silver after sale 286,266,263
For some players the silver differences might be material, but if I start playing regularly again I think I might just about have enough.