This is one example of a French import into the English language. It is , however , in common use an may be regarded as "naturalized".

For "enveloppe" by itself. The company I formerly worked for as an in-house translator preferred "spending envelope", referring to monies set aside in a budget. That's how I've always translated it. It can also mean "total dollars", "part of the budget earmarked for...". But "envelope" is a perfectly acceptable financial term.

It's common parlance in Canadian political and financial English; one says "the social envelope" to mean all dollars budgeted, by different departments, for "social" programs; we take comfort in the fact that English is importing from French, in a country where we still write "cheque" but we say "chequer" to mean "verifier" (lost the accents!)
Cheers

I. From Termium [note the comment at the end that I have set off with **]: English:Public Sector Budgeting Government Accounting envelope s CORRECT,OFFICIALLY APPROVED
resource envelope s CORRECT,OFFICIALLY APPROVED DEF - ... the level of resources available for each policy sector in the current year, the upcoming year and each of three planning years. Envelopes are established on an annual basis in the Fiscal Plan. They are defined primarily in terms of direct outlays from the Consolidated Revenue Fund but also take into consideration the implications of non-cash transactions such as loan guarantees and tax expenditures. s OBS - Envelope; resource envelope: Officially approved by the Management Committee for the BCF (budgetary, accounting and finance) Glossary. **Use of these terms is mandatory in Canadian government texts.** s 1998-11-12 [II] From Termium: English:Financial and Budgetary Management budget allowance s CORRECT French:Financial and Budgetary Management enveloppe budgétaire s CORRECT,FEM enveloppe s CORRECT,FEM DEF - Crédits affectés à chaque unité administrative pour sa gestion. s [III] From LGDT: Domaine(s)
économie politique et sociale finances publiques envelope (a)
Note(s): Les termes «envelope» et «resource envelope» ont été uniformisés par le Comité de ((terminologie du Secrétariat d'État)). (c) Synonyme(s): resource envelope (ab) enveloppe n f (a)
Niveau des ressources dont dispose chaque secteur de dépenses ((établi par des)) limites fixées aux crédits pour un programme déterminé durant la période donnée, ce qui n'exclut toutefois pas la possibilité de modifier plus tard la répartition interne des crédits. (ab) Note(s): Les termes «enveloppe» et «enveloppe budgétaire» ont été uniformisés par le Comité ((de terminologie du Secrétariat d'État)). (c) Synonyme(s): enveloppe budgétaire n f (ab)
[Office de la langue française, 1997] RBT8393128 [IV] From LGDT: Domaine(s) gestion gestion financière budget package (a) enveloppe budgétaire n f (a) Crédits budgétaires prévus pour couvrir une activité pendant une période déterminée. (a)
Synonyme(s): enveloppe n f (a) [1985] RBT8419974 [V] Hee is another one from LGDT, without any English translation. In this case, especially, I should think that **package** would be the best way to handle the French "enveloppe": Domaine(s) gestion
gestion financière enveloppe n m (a) Montant global des crédits affectés à une mission déterminée et répartis entre les différentes administrations et organismes chargés d'en assurer l'exécution (ex.: l'enveloppe recherche, l'enveloppe formation professionnelle). (a) [Office de la langue française, 1980]

It is as you say. It is often just budget, but could also be allocated funds (at least in US E).

Kirk