When two people not only agree on a sale in theory but also make a valid kinyan (act of acquisition), neither side can back out of the deal. When money is paid for a movable object but no valid kinyan is made, it is possible for either side to back out. Nevertheless, whoever backs out is subject to a severe, curse-like sanction, known as a mi shepara.[1] If each side only gives his word, R. Yochanan and Reish Lakish disagree as to whether there is a moral obligation, known as mechusar amanah (lacking credibility), to go through with the sale.[2] We accept R. Yochanan’s opinion that such an obligation exists.[3] At first glance, we would say that if you agreed to buy the sefer, you should follow through, but if you just indicated that you would likely buy the sefer if he receives it, you need not.
However, there are other halachic factors to consider. For this investigation, we presume that the parameters of mi shepara and mechusar amana are similar. Perhaps mechusar amanah applies only in a case in which a kinyan was possible but was not yet carried out. In your case, in contrast, the storeowner did not own the sefer and thus ostensibly could not transfer it, and the matter might thus be too far removed from a kinyan for any obligation to exist. The Rosh[4] seems to present precisely this point. In contrast, the Rambam[5] asserts that if one paid for an item that the seller does not yet possess and it has a set price, the seller must acquire it on the buyer’s behalf or be subject to a mi shepara. The S’ma[6] explains that this refers to a situation in which the object is readily attainable, and the seller’s commitment to see the deal through can therefore be expected to be sufficiently serious.[7] The Shach,[8] though, writes that there is always a mi shepara after paying for an object, even if it was not readily attainable at the time. It sounds like the sefer that you are looking for is hard to come by. Therefore, if you agreed to buy the sefer, whether the agreement created an obligation or not would depend on these machlokot.[9]
There is also a machloket regarding whether it is mechusar amanah to back out when the item’s going rate changed after the agreement.[10] The logic of the lenient opinion is that the moral obligation on someone to keep an oral agreement applies only when there is no compelling reason to back out of it. According to this view, one could deliberate whether your need to obtain the sefer without delay is a comparably compelling excuse to back out of the agreement.
There may be another reason to exempt you from going through with the purchase. If you did not agree on a price, then irrespective of issues of availability, the agreement is not yet complete, and it is not mechusar amanah to not go through with the purchase.[11]
A reasonable question is whether you must compensate the storeowner for buying the sefer based on your request. It is far-fetched to say that he was acting as your agent if and when he bought it. However, spending money based on another’s assurance may at times obligate the assuror to compensate. For example, if Reuven asked Shimon to make something for him, which he did, and in the event that Reuven does not take it the object, it will cause Shimon loss, Reuven has to pay for the loss Shimon incurred.[12] It is far from clear that this applies in your case, however, because if there is a market for the sefer, it likely will not be a loss for the storeowner to order it, even if you do not buy it.
In summary, you are not required to buy from the store that intended to order the sefer for you. Nevertheless, all things being roughly equal, you should try to keep your word, or at least inform the storeowner not to order it for you (as it might not be too late), apologize, and allow him to raise a grievance that perhaps we are unaware of.