Sun daan is given on Sunday: wheat, jaggery, copper vessels, red flowers. Charity addresses a graha without amplifying it, which is why it is prescribed where a gemstone is withheld.
Daan is charity aimed at a specific graha, given on its day and matched to its significations. Classically it is the remedy that works when a gemstone would be dangerous — it addresses the planet without amplifying it, which is why it is prescribed for Rahu and Saturn where stones are withheld.
Sun charity goes to fathers and father-figures, and to organisations supporting elderly men — given on Sunday.
The corpus describes an ill-placed Sun as low self-esteem, absent energy and confidence, and friction with authority and with the father. That is the pattern these practices address.
Sunday. Fasting, charity and mantra japa for Sun all belong to that day.
The mantra, the weekday fast and the charity are safe for anyone — they address Sun without amplifying it. Ruby is not: a planetary gem strengthens whatever the graha is already doing, so it needs a chart reading first.