Works:
<field name="expression">balp[][('tax_ids.tag_ids.id', '=', ref('l10n_ee_accounting.kmd2017_3_2').id), ('tax_ids.type_tax_use', '=', 'purchase')] - balp[][('tax_ids.tag_ids.id', '=', ref('l10n_ee_accounting.kmd2017_3_2').id), ('tax_ids.type_tax_use', '=', 'sale')]</field>
Same but multiline does not:
<field name="expression">
balp[][
('tax_ids.tag_ids.id', '=', ref('l10n_ee_accounting.kmd2017_3_2').id), ('tax_ids.type_tax_use', '=', 'purchase')
] - balp[][
('tax_ids.tag_ids.id', '=', ref('l10n_ee_accounting.kmd2017_3_2').id), ('tax_ids.type_tax_use', '=', 'sale')
]
</field>
Works:
Same but multiline does not: