The Concom and especially the Treasurer should establish a consistent (and transparent) policy for how convention related expenses will be paid to individuals and vendors, and make it known widely and clearly. In the case of individuals spending their own funds and looking to the convention for reimbursement, the Treasurer should establish exactly what documentation should be provided before payment is made. At a minimum, the individual purchaser should provide receipts for the items purchased, and some description of the reason and authorization for the purchase.
For a large convention operation, a more extensive process may be developed. In addition to receipts, the individual may be required to document the date, item, and price of each purchase, and which department budget or convention purpose the expense should be set against. Standards for some kinds of expense ("bulk items") may be set and a narrative justification may be required if the expense exceeds the standard. A form of some kind may be provided for the purpose, to help with the bookkeeping and filing by the Treasurer.
It is important to remember that the convention is a volunteer organization. On the one hand, it is unreasonable to expect all volunteers will simply support the convention in all ways from their own personal funds. On the other hand, the convention must very carefully sheperd its funds or it will find itself bankrupt very quickly. Disputes can be avoided by publishing the rules and policies early and obviously.