The 18-year-old joined the Champions League qualifiers for a fee reported to be worth up to £3million but Sporting immediately slapped a £47.8million buy-out clause in the midfielder's six-year contract.
United could be set for a more immediate additional windfall amid reports they have rejected a £1.5million bid from West Ham for 20-year-old left-back Andy Robertson, who made his Scotland debut last season after moving from amateur side Queen's Park last summer.
And future riches await them if playmaker Gauld fulfils his potential.
Chairman Stephen Thompson told Sky Sports News: "It's never easy to to let one of your best players go but it was a fantastic deal for the club and a great opportunity for Ryan
"I cannot discuss the confidentiality of the agreement with Sporting but we are very pleased with the contract that we have got
If he is successful and moves on further down the line then we will benefit from that as well
"We have said for a number of months that we didn't want to sell anybody unless the deal was right for our club and it was the right club for Ryan
Once he found out who the club were he was very keen to go and speak to them.
"They were the first club to come to the table and have proper negotiations with us
He is a pretty quiet lad, he will get on with it and I think he will settle in well
It's a great opportunity and a great club that he has gone to."
Gauld made 50 first-team appearances for United, during which time he had to cope with reports that Real Madrid and Manchester United were watching him, among other clubs.
But he remained grounded thanks to the nurturing of manager Jackie McNamara and the support of his team-mates - he shared a flat with the likes of Robertson and fellow teenager John Souttar
Thompson said: "I think he has handled the pressure over the last year very well and I think he will continue to handle it very well
He's only 18 but he's very mature for his age."
Thompson refused to comment on reports about West Ham's interest in Robertson, who was an ever-present last season and is also understood to be interesting Everton.
But transfer talk will remain prevalent at Tannadice with the likes of Souttar also attracting attention and McNamara looking to strengthen his squad.
"It puts us on the map in Europe and we have a lot of young talent at the club - hopefully they can kick on and have another successful season," Thompson said.
"We have carefully structured our finances at this club
Jackie and I were chatting this morning about players we would like to bring in for next season and we will keep working quietly towards that."
Source : PA
Source: PA