Under Law 41.3 of the Laws of Cricket (governed by the MCC), players may legally maintain the ball by:
- Polishing it on clothing, using only sweat (no artificial substances).
- Drying it with a towel if wet.
- Removing mud under umpire supervision.
All other actions that alter the ball's condition are illegal, including deliberately rubbing the ball on the ground or using external substances (natural or artificial) beyond permitted methods. Sources like Wikipedia and ESPN Cricinfo explicitly list "rubbing the ball on the ground" as a prohibited form of tampering.
The intent here—picking up grease or dirt/moisture from the grass to transfer it to the ball—would count as unfairly changing the ball's condition, similar to past incidents involving foreign substances (e.g., mints for sugary saliva or dirt in pockets). This could result in a 5-run penalty, ball replacement, fines, or bans if proven.
Normal fielding might accidentally get grass or dirt on the ball (which umpires can have removed if needed), but deliberately collecting it on hands for application crosses into unfair play. Players shine the ball legally with sweat on clothing only.