How can one repent from taking aluminum without permission from a public company that has merged with others, and should the money be returned at the past or current price, or should it be given as charity?

Taking aluminum from the company you worked for is forbidden, as it is taking money unjustly and a betrayal of trust. Your delay in repenting is a grave dereliction. Along with repentance, you must now return what you took, or if that is not possible, return its value on the day of the theft. Since returning the value to its owners is difficult due to the company's merger, you must spend this value on charitable causes and for the benefit of Muslims.