What is the ruling on betting that does not involve money, such as stipulating shaving one's mustache for the loser?

It is not permissible to engage in the aforementioned wager, as it is a forbidden wager and a prohibited form of gambling (maysir). The fact that no money is involved does not make it permissible. Maysir includes all types of risk-taking, whether with money or without. Jurists have divided maysir into maysir of amusement (without money) and maysir of gambling (with money). This wager generates hatred and animosity, as Allah Almighty says: "Satan only wants to cause animosity and hatred among you through intoxicants and gambling and to avert you from the remembrance of Allah and from prayer. So will you not desist?" The reason for the prohibition lies in the corrupting elements mentioned in the verses. If these corrupting elements are found in a transaction, even if it does not involve money, then it is forbidden. The mustache should be trimmed, not shaved completely.