First ḥadīth
Umar (may Allah be pleased with him) said: “I donated a horse in the cause of Allah. The one who received it neglected it, so I wanted to buy it back, thinking he would sell it cheaply. I asked the Prophet ﷺ about that, and he replied, ‘Do not purchase it, and do not take back your charity even if he were to give it to you for a single dirham, for whoever retracts his gift is like one who returns to his own vomit.’

In another wording: ‘The one who takes back his charity is like a dog that vomits and then returns to its vomit.’

Second ḥadīth
Ibn ʿAbbās (may Allah be pleased with them both) reported that the Messenger of Allah ﷺ said: “He who takes back his gift is like one who returns to his own vomit.”

Overall meaning
Umar ibn al-Khaṭṭāb (may Allah be pleased with him) helped a man who was going out to fight in Allah’s path by giving him a horse to ride. The man, however, spent nothing on the animal, failed to care for it, and overworked it until it became thin and weak. Seeing its condition, Umar wished to buy it back, knowing it would now be inexpensive. Before doing so he consulted the Prophet ﷺ, for his inspired heart felt uneasy about the matter. The Prophet ﷺ forbade him to purchase it, even for a token price, because what is offered for Allah’s sake should not be pursued again, nor should one’s heart become attached to it. Accepting a favorably low price could amount to re-claiming part of one’s charity. Moreover, since the act had already erased sins and purified him from spiritual impurities, it was unfitting for it to return to him; hence the Prophet ﷺ described buying it back as “returning to one’s charity.”

To deter anyone from such conduct, the Prophet ﷺ used the most repulsive image: a dog that vomits and then comes back to eat its vomit—an image that underscores the ugliness, baseness, and ignominy of the deed.

Lessons drawn from the two ḥadiths

1. It is recommended to support those who fight in the path of Allah; such assistance counts among the finest forms of charity, and the Prophet ﷺ explicitly called it “charity.”
2. Umar’s gift of the horse was a transfer of ownership, not an endowment restricted to jihad. Had it been a waqf, the man would not have been permitted to sell it. The intention, therefore, was a permanent gift of possession, not a conditional endowment.
3. It is forbidden to buy back one’s charity, for what has been given for Allah’s sake should not attract one’s desire afterward. Purchasing it indicates renewed attachment, and the seller may favor the original donor with a discount, allowing some of the charity to revert to him.