235. " there is no sin on you if you make an indirect proposal of marriage
to (such) women, or keep concealed (the intention) to yourselves;
allah knows that you will think of them;
but make no promise to them in secret, unless you speak in fairness.
and do not resolve on a wedlock until the prescribed period is completed,
and know that allah knows what is in your hearts,
so beware of him; and know that allah is forgiving, forbearing."

commentary:
can widows be engaged during waiting period ?

as to the foregoing qur'anic discussion about the waiting-period
of women after their husband's death, in this verse, one of the important
ordinances in relation to the widows in their waiting-period is pointed
out. it says:

" there is no sin on you if you make an indirect proposal of marriage
to (such) women, or keep concealed (the intention) to yourselves;
allah knows that you will think of them;
but make no promise to them in secret, unless you speak in fairness. ..."

it is, in fact, a natural phenomenon that a woman may think of
her future fate after the death of her husband. and, on the other hand,
some men may also be in thought of marriage with them because of
easier conditions that widows usually have.
then, continuing the subject, the verse says:

"... and do not resolve on a wedlock until the prescribed
period is completed, ..."

it is certain that if a person confirms a wedlock which is during
the waiting-period, it is totally invalid /batil/. moreover, if he does it
intentionally, that action causes the woman to be unlawful /haram/ to
him forever.
then, it follows saying:

"... and know that allah knows what is in your hearts,
so beware of him; and know that allah is forgiving, forbearing."

that is, allah does not hasten to punish his servants, because
he is ` forbearing ' about what they do.