Like all function fields, it must return a dictionary with an entry and value for every ID you get passed in ids, although your value can be False, None, []
In your case your functional field is declared as a one2many type which means your functional field must return a dictionary with an entry per id and the value, a list of integers that represent the ids of the related table, in your case, property.expense.
A very common pattern is:
def _property_expense_preset_expenses(self, cr, uid, ids, field, arg, context = None):
res = {}
for spu in self.browse(cr, uid, ids, context = context):
res[spu.id] = []
for preset in spu.property_expense_presets:
res[spu.id].extend([x.id for x in preset.expense_preset.expenses])
return res
Assuming ids contains 1,2,3 you will get a result of {1: [...], 2: [...], 3: []}
Where each list contains the integer ids of the expenses or an empty list if there are none.
As a general comment, I note your code doesn't default the context argument to None or pass the context as a named argument to the browse method - it is important to do both.