Learn Russian: Сострадание важнее, и мы не можем не пожертвовать деньгами. - Compassion is more important, and we cannot help but sacrifice the money.

This is the classic не + conjugated мочь + не + infinitive structure. Literally, it means "we cannot not sacrifice." In Russian, this double negative creates a strong affirmation or a sense of inevitability, translating smoothly to "we must" or "we cannot help but."

Why is деньгами in the instrumental case? Shouldn't the thing we sacrifice be the direct object in the accusative case?

The verb пожертвовать (and its imperfective counterpart жертвовать) strictly requires the instrumental case for the thing being donated or given up. You can think of it conceptually as making a sacrifice with the money.

Why use the perfective verb пожертвовать in this double-negative construction rather than the imperfective?

The choice of aspect here depends entirely on the context of the action. We use the perfective пожертвовать because the speaker is talking about a specific, one-time decision to give up this money, rather than a general, ongoing habit of donating.

Why use важнее instead of более важное for "more important"?