I was reading these two suttas and at the the end of both the monks seem to leave although Citta has offered to care for them. Is this a rejection of his Dana? Did Ven. Mahaka do something wrong by his display of concentration?
It should be not understood as "rejection" or disprove if offered isn't made use of, good householder, can have many reasons, often Vinaya. Ones act of merits (if meritorious intended) within such giving still presist and not touched by the receivers circumstances, deeds on it.
Noble Ones (as well those understanding Vinaya an following it) do not receive gift in return of having given Dhamma, their assistance, good householder. They avoid feeding ideas of interests in nourishment for relations. And they act in ways so that one hardly could say that they assist for a livelihood, good householder (especially the psychic power case would easy lead to bad reputation if accepting rewards, yet, because other dangers forbidden to provide such at all. Easy to be called "making favors (even for gain)" ). So it's general good and importand to make gifts in advance if wishing to do such.
Receiving a meal, for excample, after having taught the Dhamma, would be a transgression of the monk's rules. But the matter goes of course even deeper.
That's by the way the reason why common consumer hardly ever would perform high Dana with great fruit.
A giver who is actually not free from corruption gives to bind, if he gives in advance, suggest things to be "free" and the fools get trapped, of course not unwilling and innocent at all, as the pig searches for it's holder, disire for birth feeds death.
[Note that this isn't given for stacks, exchange, other world-binding trades, but for an escape from this wheel]