Can you keep boiled condensed milk in tin?

That wouldn't be a good idea. You have to remember that you've introduced a significant amount of heat to both the metal and the liquid thus changing their ability to coexist in harmony. There is a reason that one is told to store tinned food in a dry cool place and to discard if overheated.

If you must do it ahead just store the milk in another container in the refrigerator. Otherwise its a health hazard waiting to happen.

Tins aren't made for heating and storage of food. Some chemicals or metals may leach into the food, if you use the tins this way. It's best to open the tins just before you use the food in them.

Opening the tins usually introduces some bacteria and fungi into the food. And these microorganisms can spoil the food, even in the fridge, if you store the food for a few days.

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