The gesture came late in Sunday's match at El Sardinero, with Real Madrid leading comfortably after goals from Kylian Mbappé, Jude Bellingham and Diomande had put them three ahead before half-time.
As the clock approached the 85th minute, Madrid prepared to make their final substitution. Diomande's number appeared on the electronic board, with Endrick waiting to replace him.
Rather than cross the pitch towards the nearest touchline, the forward was positioned on the opposite side of the field. With the match effectively decided, he chose to leave by the closest boundary and walk around the perimeter towards the dressing rooms.
But his departure soon became the focus of attention in the stands.
A young Racing Santander academy supporter had spotted the opportunity and began shouting for Diomande to give him his shirt.
"Diomande, give me your shirt! Diomande, give me your shirt!" the youngster repeatedly called.
Initially, the Madrid player appeared to continue walking without responding. He then stopped, turned around and looked towards the section where the boy was standing.
Diomande made his way over to the young supporter, who was positioned in the front row of the stand opposite the directors' box, and handed him his shirt.
The youngster was left clutching his prize, while those around him responded with a spontaneous round of applause for the forward. The ovation spread through that section of the stadium, with other spectators joining in after realising what had happened.
It was an unexpected moment of warmth in an afternoon that had otherwise been difficult for the home supporters. Racing had gone into the interval three goals behind and faced the prospect of a heavy defeat on their own ground.
Madrid had taken control early, with Mbappé converting an 11th-minute penalty before Bellingham doubled the advantage in the 21st minute. Diomande completed the scoring in the 36th minute, leaving Racing with a mountain to climb.
The final score remained 3-0, but Diomande's decision to stop for a young supporter ensured the match offered something beyond the result.
For the Racing academy player, the memory of the afternoon will not simply be the defeat or the three Madrid goals. It will be the moment a visiting forward heard his appeal, turned back and made his day by handing over his shirt.