Learn English – Usage of “but then”

What you learned there is absolutely correct. Most of the time, we do use the preposition in after the verb invest. The preposition into is also possible, but in is used a lot more commonly. However, the to that you're talking about there is not a preposition at all. It's something called an infinitive marker which goes along with the verb introduce and indicates that the verb introduce is being used in its infinitive form (for example, to buy, to see and to watch are all the infinitive forms of the verbs buy, see and watch respectively). One of the goals of using verbs in their infinitive forms is to specify the purpose of an action. For instance, to see in I went there to see the man specifies the purpose of the action of going there. I went there for the purpose of seeing the man. Consider this sentence:

They invested lots of money in the company to make even more money.

Now, I will ask you the following question what was the purpose of them investing lots of money in the company? The answer you should give would be the purpose of them investing lots of money in the company was to make even more money. Do you see how this works?

So, we could rewrite your sentence like this:

However, last year, my university invested a significant amount of money (in its infrastructure) for the purpose of introducing a greater number of healthy food options at its cafeterias.

You are right. But means only in this context. And this use of but makes the sentence formal. Here are a few examples:

We can but guess at the extent of the problem. = we can only guess...

Several Japanese companies already operate in the UK – Nissan, Honda, Fujitsu, to name but a few. = ... to name just a few

Lots of interesting different flavours combine - mixed spice and wild berries to name but two.