Is informing the mosque worker about finding unknown money with the intention of donating it without anyone's knowledge considered a lie and forbidden?

May Allah reward you for your eagerness to do good, but you have erred by lying to conceal your deed. You could have used allusions (ma'areedh). If you repent to Allah and accustom your tongue to truthfulness, Allah will accept your repentance, and the reward of your charity will remain stored for you. Note that your failure to clarify to the mosque worker that the money is specifically for the mosque might lead him to spend it on lost and found items or money whose owner is unknown, thereby missing your intended purpose of donating it to the mosque.