The retrospective sketch of The Singing Butler – Vettriano's most iconic painting – was sold to a private UK buyer for £32,000 at the Sotheby's sale at Gleneagles on Tuesday night.

The addition of a buyer's premium by Sotheby's took the total sale price to £39,650, although the St Andrews University student charities campaign who put the work up for auction will receive the hammer price of £32,000.

The sketch, completed by Vettriano during a short master class he gave to St Andrews students in 2003, had previously been valued at £20,000.

The artist and the university principal, Dr Brian Lang, agreed the work should be gifted to the student charity effort, which last year raised a record £160,000.

The original Singing Butler, which sold for £744,800 in 2004, still holds the record for any painting sold at auction in Scotland.

Nervous student representatives watched from the wings as the bidding began at Gleneagles on Tuesday.

"There was a real sense of anticipation in the room when the bidding opened – no-one was quite sure if we were about to discover that the credit crunch had hit the art market," said a university spokesman.

"As the bids rose our smiles got broader and we were absolutely delighted when the auctioneer finally brought his hammer down at £32,000.

"It's a fantastic boost for a student charities campaign which has already proved to be one of the most active in Scotland.

"We're very grateful to Jack for the support he's given students and student charities in St Andrews over many years."

Charities that will benefit from the sale include Unicef, Breast Cancer Campaign, Refuge, Joy to the World, Chest, Heart and Stroke Association and Alzheimer's Scotland.