English to French Law/Patents Real Estate

The Assurance to the Buyer shall be in the form of the annexed draft lease. The Assurance shall be engrossed in duplicate by the Seller's Solicitors and a counterpart shall be executed by the Buyer before the date for completion and the Buyer shall contribute £100 plus VAT towards the Sellers costs thereof.
In the event of any variation to the plan annexed to the assurance being required after completion arising out of any requisition raised by H. M. Land Registry then the parties will agree to a revised plan being annexed to the assurance and execute the same but so that the area and or use and enjoyment of the Property is not substantially diminished and any such variation shall not give rise to a claim for compensation.

Je n'ai pas de définition pour ce terme dans mon document. (vente d'un appartement mais avec un bail)

Dans des Standard Conditions of Sale de la UK Law Society je trouve ceci : “Assurance” means the document or documents whereby the Sale is to be carried into effect

S'agirait il d'un "Acte de transfert de propriété "?

Merci de votre aide, je n'ai jamais rencontré ce terme et mon texte n'aide pas beaucoup.